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Author Topic: Washout in CLG's  (Read 1799 times)
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Sailaway
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« on: April 23, 2016, 07:33:35 AM »

I am coming back to CLG after a  long time. Just built the Straight-up ( Free plans in AMA magazine) and enjoying shooting it straight up. Can you guys clear some concepts on "washout" for me. This model does not ask for a washout on either wing tips. And it does well ( 45 seconds at early dawn).

So is Washout a good idea on all CLG ?

The plans always show a bit of Off-line cut at the tip (usually 1/16) . I get the cut exactly right but do not see the washout when I glue it quite congruent at the seam. So when it says 1/16 inch cut, does that mean I should expect 1/16 washout at the end of the tip? On the other hand would I be better off sanding in washout at the tip before I carve in the highpoint airfoil on top ? Sand in 1/16 washout at bottom instead of the off line cut ?
I am under the impression that you need more washout on calm days and very little on Windy. Is that even correct? So most of you build two CLG's at same time, one for windy and one for calm days? Currently I build only a single CLG but move CG and decal age on windy days ( CG in front a bit) and on calm days CG back.

Thanks in advance to clear my concepts as what is this all about .
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Rewinged
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« Reply #1 on: April 27, 2016, 03:35:49 PM »

I'm surprised nobody has responded to this.  I saw it earlier and forgot to respond.  Some quick comments:

Some washout is always valuable.  I don't think anybody specializes in washout for windy vs. calm days, although I think it likely is relatively valuable on a windy/more turbulent day than a calm day.  We plan to launch into thermals, which almost by definition can have a bit of turbulence, although some are pretty benign.

With an offset polyhedral joint you will not see the washout looking at the tip as you are, since the whole tip is washed out uniformly.  It can be better to have a slightly gradual sanded-in washout, but that is a very fine distinction, and if not done right it can make things worse instead of better:  Despite knowing how to do that, I messed up on 2 glider wings I made last summer, and got too much washout as a result.  Climb and glide both suffered.

Moving CG as you do, very slightly, and perhaps adjusting decalage slightly, is something I also do.  But pretty minor changes.  Too little decalage risks a spin-in in a thermal.

Regards,
Bill
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USch
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« Reply #2 on: April 27, 2016, 05:18:50 PM »


......I am under the impression that you need more washout on calm days and very little on Windy........


My 2 pence worth...
Washout has nothing to do with calm or turbulent air. Otherwise how comes also indoor gliders use it? Actually indoor flappers have a lot of them. A Slow Poker has a piece of balsa with the grain running chordwise to stiffen the wing tip and in the center the flap goes down at least 1/4" (or more) and this gives a lot of washout. Same thing if you look at the WIF 5 and 7 from Bill Gowen. He stiffens the tip running a carbon roving over the forward part of the wing AND the flap section with the same result.

Washout has to do with flight stability. If you have to adjust for calm or turbulent air do it by moving the CG as Rewinged suggest, more forward for faster glide in turbulent air. But in very small amounts. The same can be done by altering the flight circle diameter, tighter on turbulent days, wider in calm air. Wider circle is the same as increasing the incidence, getting nearer to the stall. My personal view dictates that on a well trimmed model you should not touch the CG but only the relative incidence between wing and tail.

About the skew on polyhedral wings. It is a far more precise way to get the right amount of washout than sanding the underside of the wing plank. Somebody may object that a continuous sanded washout is more efficient. But this theory has to be proved and I think no expert will confirm a difference between the one or other way of putting washout on a wing.

Actually I see another advantage of the skewed joint. If you sand 3/32" of skew on the left hand joint and 1/16" on the right hand joint you get both worlds, washout on both wing tips AND washin on the right wing. Washin on the right wing helps to keep up the wing in the climb and to centre in thermals.

Urs
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Olbill
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« Reply #3 on: April 27, 2016, 05:40:47 PM »

After reading the above about skewed joints and tip washout I decided to make another stab at understanding how this works. So I looked at all the plans I could find that showed skewed joints. All I came up with is more confusion since there were several different methods of doing the skew on the plans I looked at. It makes me glad I have simple dihedral in my gliders!

As far as tip washout in my own gliders, what Urs says is true except that the airfoil on my Cat 2 UCLG doesn't flatten out. The curl in the flaps continues all the way to the tip plates. I can't remember about my Cat 2 SCLG and will have to find it to figure it out.
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Rewinged
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« Reply #4 on: April 27, 2016, 09:13:17 PM »

The venerable Stan Buddenbohm used sanded washout on his record-setting high ceiling indoor gliders.  They tested many things in preparation for the record attempts at Tustin, and used sanded washout.  So that is a true expert who will confirm a difference.

Related, perhaps:  The honorable Leeper once told me I had the highest-launching outdoor CLG he had ever seen.  Truly, he was buttering me up, no doubt, but there is a difference between skewed and sanded washout, done properly.  My theory is that, well, it's my theory and definitely can't be proved or disproved online, but it has to do with flow separation at the lower airfoil "chin" during the launch.  A proper coupling of washout and airfoil, graduating to a semi-symmetrical airfoil at the tip, provides a higher launch without hurting the glide.

There are ways of getting the sanded washout every bit as accurate as an offset joint.  A bigger issue may be optimizing the amount of washout, whatever the method.

For outdoor flying, what really matters is launching into a thermal.  Higher helps, but air-picking is of much greater importance.
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USch
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« Reply #5 on: April 28, 2016, 06:11:48 AM »

Rewinged,
probably I was a little to fast typing my reply and should not have used the word "theory".
What I actually wanted to express was that for 99% of CLG and HLG flyers the eventual differences between sanded and skewed washout is not noticable and, as you said, thermal picking and a well trimmed airplane is far more important for good flights. And sanded washout has to be combined with change of section with less mean camber, not just sanding away on the bottom (as you already pointed out).

Interesting your remark
...but it has to do with flow separation at the lower airfoil "chin" during the launch.
What is chin in this context ?
Please help a non english motherlanguage guy to understand. Even with google translater I do not get to grips with this one  Sad

Urs

PS, I would like to hear the opinion of Leeper, the inventor of skewed tips, about his experience.
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Rewinged
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« Reply #6 on: April 28, 2016, 12:21:05 PM »

"thermal picking and a well trimmed airplane is far more important for good flights."

Sailaway, what Urs had to say there is key.  Using the washout shown on good plan, like for the Straight Up, should give you the potential for a very, very good flying airplane.

Urs, about my theory: As we know, washout reduces the angle of attack in the washed-out region.  During the high-speed launch, most of the wing is trying to fly close to zero angle of attack.  The tips, then, are at a slightly negative angle of attack.  My theory is that, dependent upon airfoil, if the leading edge of the airfoil is tipped down--as it is with skewed tips--then the air can't make it around the bottom of the leading edge (the "chin") without separation. The resulting flow separation causes drag.  Obviously, this is dependent upon the amount the leading edge is tipped down and how sharp the leading edge is.  So, even with washout from a skewed joint, this can probably be handled by simply rounding the leading edge a bit.  My recollection of what Buddenbohm has said or written is that he maintains the same leading edge toward the tips, but all thinning is done on the top to become more semi-symmetrical  I hedge my bets--since I don't know how much washout is optimal, I also round the leading edge toward the tips.  That is, when I don't mess up badly, like I did last year.

Yeah, where is the venerable user of all things in moderation?  I've "seen" him online recently, but not here.  Leeper?

--Bill

P.S.  Urs, it is impossible to tell that English is not your "mother language."  I'm constantly embarrassed that most people in the U.S., including me, have almost no capabilities with other languages.  
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DerekMc
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« Reply #7 on: April 28, 2016, 12:40:24 PM »


P.S.  Urs, it is impossible to tell that English is not your "mother language."  I'm constantly embarrassed that most people in the U.S., including me, have almost no capabilities with other languages.  

Yo tambien Grin
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Derek
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« Reply #8 on: April 28, 2016, 02:38:14 PM »

Bill, in the USA you have the fortune  to have a lot of model magazines around. When I started model building there was only one German magazine, so it was unavoidably to read Aero Modeller in English. And now, 50+ years after that, I have a English lesson each day reading HPA  Grin

Derek, your Spanglish is excellent  Wink

Back to serious matters. Now I understand what you mean with chin. Give me some time and I will try to write why I am not convinced about this argument.

Leeper, we are still waiting for some wise word  Roll Eyes

Urs
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Hepcat
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« Reply #9 on: April 28, 2016, 10:56:52 PM »

Many years ago If a piloted aeroplane stalled nasty things could happen and it was found that washout gave the pilot a bit of warning of a stall.  As FF aeroplanes don't have pilots you can forget that.  Also it was found that washout could give even chord aeroplanes a better lift distribution and less drag.  However as CLG builders can make the wing what shape they like you can forget that.
 
Now we all know that subtle warps on flying surfaces are at the very heart of trimming FF aeroplanes but that is almost always asymmetrical warps.  So, what does washout on both tips do?  Well, fundamentally it makes the aeroplane less longitudinally stable.  It is a basic fact of stability that an aeroplane must have some longitudinal dihedral (decalage).  Reducing all, or part, of the wing incidence will reduce stability. Now I know that CLGs are a class that depend on fine adjustment of longitudinal stability but to me a change in wing or tail incidence seems more reliable and repeatable than sanding away bits of the tips.

I do not think that the step change in incidence at a skewed joint is a good thing but if you want to persist the necessary calculation is as follows:

W is washout angle, D is dihedral angle of tip relative to inner panel, X is the offset of the cut at the trailing edge, C is the wing chord.  Then W = atan((X/C) tanD).
Example: If C=3, X=0.062, D=20 degrees then the washout would be 0.43 degrees.

However remember wise Old Bill, never over rate the math.  Don't forget skewing the cut will sweep back the tip and then where do you measure the dihedral?

John

   








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USch
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« Reply #10 on: April 29, 2016, 02:28:10 PM »

Dear Hepcat, I dont want to challenge you on pure theory because I am an evidence based modeller. That means I always have a close look what other, more prepared persons, do and design in there models plus how my models behave in the air.

Now even a highly recommended aerodynamicist like Mark Drela is using washout on his RC gliders (from 0,5-1° depending on the model). His wings have a planform approaching the ellipse but changing airfoil section (thinning) from center to tip. His explanation for washout is a possible tighter turn in thermals. Now you may argue that these are RC controlled gliders enabling larger or tighter turns via control input.

But also FF models do not have a steady circle in flight. With the help of wing warps and other asymmetries a good model will fly in larger circles in neutral to bad air and tighten the circle once found a thermal. My personal evidence shows that a bit of washout on certain models helps in more resistance to stalling, better stall recovering and generally better performance.

My personal thought about all this is that washout, in the right amount, is to equalise the zero lift line of the various airfoil sections along the span. As normally on HLG or CLG gliders airfoil section is getting thinner, percent wise, towards the tip.

Urs
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« Reply #11 on: April 29, 2016, 05:27:06 PM »

So, what does washout on both tips do?  Well, fundamentally it makes the aeroplane less longitudinally stable.  It is a basic fact of stability that an aeroplane must have some longitudinal dihedral (decalage).  Reducing all, or part, of the wing incidence will reduce stability. Now I know that CLGs are a class that depend on fine adjustment of longitudinal stability but to me a change in wing or tail incidence seems more reliable and repeatable than sanding away bits of the tips.
For be it for me to disagree with Guru Hepcat ...  Shocked

But surely washout on both tips just reduces wing incidence which results in a more 'diving' trim.  Whether or not stability is affected depends on how this 'diving' trim is cured.

If the CG is moved back, then stability is indeed adversely affected but this is due to the more backward CG rather than washout.

Instead, if the tail is breathed upon and massaged up (up elevator) then decalage is 'unchanged' and longitudinal stability should not be affected.  Grin Not sure if this is more repeatable or reliable  Cool

What IS affected is spiral (?) stability as the washout means the wing tips are less likely to stall before the centre-section so increases stability.  Tip stall will always happen on one side before the other and may lead to spinning if other factors affecting spiral stability are iffy .. eg lack of dihedral and backward CG

Spinning (and spin recovery) was poorly understood until well after WW1.  Nevil Shute's early novels, written in those days, have a couple of good descriptions of the fearsome reputation of this evil and what happens.
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« Reply #12 on: April 30, 2016, 10:02:11 AM »

Urs,
I am sorry you thought I was delving deep into theory when I was trying to introduce a little humour at the expense of those of us (and I don’t exclude myself)  who drop into the trap of thinking that full size aerodynamics are always applicable to models.  Anyhow I have certainly got my ‘comeuppance’ for my frivolity with your excellent example of the usefulness of wash out when the wing section (mainly the camber) changes towards the tip.

Does anyone recall a designer called Koutney who was very successful with scale models?  Although most full size aeroplanes tend towards thick, almost symmetrical, sections at the tip he used thin sections with a lot of camber which avoided wash out.

Ricardo,
To some extent you have answered yourself.  You said washing out both tips is like reducing wing incidence and causing a diving trim.  To prevent the dive you could put negative on the tailplane which would restore the decalage.  Don’t forget that the decalage is measured between the zero lift lines of the wing and tailplane and the zero lift line of a wing could be affected by different aerofoils along the wing, warps, flaps and on the tailplane by differing positions of the elevator. (So your breathing on the tail does affect the decalage!)

There is then the question of the importance of tip stalling. I fly me FF models to the right so I always like more incidence on the starboard wing tip so that if the model approaches a stall the starboard tip stalls and the model enters a right turn which is the quick way to return to smooth flight.  If both tips stalled together the model would probably continue switchbacking for ages.  I don’t think that tip stalling is much of a problem to FF flyers since they learned a few basics and discovered dihedral.

 In the early days of piloted aeroplanes a stall often led to a spin and a crash.  The machine headed for the ground and the pilot pulled back on the stick to slow the descent.  This served to keep the ‘plane in a stall and an inevitable crash.  Small aerodynamic changes and education of pilots to push the stick forwards to regain flying speed removed the danger of spins, except near the ground of course.

John
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« Reply #13 on: April 30, 2016, 04:31:23 PM »

Hi all!
Been busy & just now saw this CLG Washout thread.
Most of you (JB, Urs, Rewinged Bill,etc)have pretty much stated indisputable comments on CLG washout & trimming thereof.
Now for my 2Cents worth: As some of the aforementioned 'lads' Grin may recall, I have said my use of tip dihedral skew started
during my early HLG building days, & continues to this day with CLGs, some 60 years hence.
I soon realised trying to build perfectly aligned joints could produce very undesirable errors. So, in my emperical mind, if I wanted
some washout (always did!) I should made the cut by skewing it, which gets the tip washout properly started. Skewed washout
dimension is not very critical, IMO, as I have used as much as .25inch with seemingly little drag effect on climb. It flew fine actually.
I always sand some washout, as I want it to be progressive toward each tip.
To my enlightened surprise, even the learned Dr Drela has said much the same about this washout subject.
I also espouse to his thinning airfoil ideas as a given, ever since I can recall!
Somewhat vindicating, eh?
Leeper, OUT!  Wink
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« Reply #14 on: May 02, 2016, 06:52:45 PM »

Thank you all so very much. This is a  hard concept for a new guy like me. So in conclusion, I will always stay with the angled (off-line) cut only to give the washout. And all my CLG will get a washout  Leeper, can you explain a bit more when you say "I soon realized trying to build perfectly aligned joints could produce very undesirable errors." In particular what is meant by "Perfectly aligned joints" that produces error? Pardon my learning curve questions.

I have built both the Straight up and Eplix6 CLG's. Now after many launches and many weekends I have learnt to trim the CLG's. I used the wood I had which was heavy ( 8 to 10 pounds). Straight Up CLG should be around 13 grams and mine is 19. But the take off, transition and flat glide just around a slight hint of stall with the left turn diameter adjusted by tip weight clay was a very big learning curve. I fly around 7 AM as no one is in the park ( including thermals) and get 45 seconds most time. Soon I will buy some contest wood. I am hooked or "Catapulted".
Regards,
Sailaway.

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« Reply #15 on: May 02, 2016, 07:42:10 PM »

Sailaway, I see that I did not explain myself correctly.
I meant to imply that if one tries as best he can to make things perfect...it is impossible, because we are mere humans.
So a slip of the knife or sanding block might cause a dihedral joint to not be correctly (read: perfectly) aligned.
Hence, it is likely the joint might be the wrong way, creating some washin or other mischief.
So it sounds as if you are well on your way to successful CLG flying & I send my congratulations.
If I may suggest, you cannot go wrong to contact Stan Buddenbohm for the very best CLG/HLG kits.
His email: gliderbohm@aol.com  Try DiscUSkid.com for his catalog.
He hand selects & cuts the proper balsa for the tasks at hand.
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« Reply #16 on: May 03, 2016, 11:21:49 AM »

Thank You Mr. Leeper. It is very clear now. I am having fun with the CLG's. I will check out at least one of Stan's CLG to follow correctly. I am very happy with his Dynomite TLG that I still fly after 3 years. If I build it right and then trim it, they do not break. So the lighter the better for more then that reason  It is beginning to come together for me now.
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« Reply #17 on: August 01, 2016, 09:14:14 PM »

This is not a response to anything recent but somebody on the FF forum asked about achieving washout by an angled joint between the center panel and the tip panel. I listed a formula for calculating the wash-out angle from the chord, the cut back of the trailing edge and the dihedral angle but I know lots of people don't like using maths at all so I wondered if I could do it without.  I found that I could illustrate the wash-out angle just by drawing three straight lines on the plan so I did a quick sketch to explain.  There are just the three red lines to draw.  First the one carrying straight on from the end of the trailing edge and equal in length to the  chord. The second line starts from the cut line at the trailing edge and slopes up at the dihedral angle and only needs to be long enough to cross the blue line.  The third red line just runs from the end of the first line to where the second line crosses the blue line and the angle between red lines one and three is the washout angle.I know it is not world shattering but I have not seen it done in quite this way before so I thought it might be useful to put the sketch on this thread where it might be useful to a beginner.
John  
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Re: Washout in CLG's
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« Reply #18 on: August 02, 2016, 09:56:18 PM »

Thanks John that is a simple check to carry out on a drawing. I'm adding it to my list of tips.

John
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« Reply #19 on: August 02, 2016, 10:09:41 PM »

Hepcat,
Love the graphic representation of the solution.
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